Search any relevant u21 squads on/after their regen dates. It is a tedious process but very much worthwhile. Here are the regen dates to help you:
26th January:-
Anguilla, Antigua & Barbuda, Aruba, Barbados, Bermuda, British Virgin Islands, Cayman Islands, Costa Rica, Cuba, Curaçao, Dominica, Dominican Rep., El Salvador, Grenada, Guatemala, Guyana, Honduras, Jamaica, Montserrat, Nicaragua, Panama, Puerto Rico, Saint Kitts & Nevis, Saint Lucia, St Vincent & the Grenadines, Surinam, Trinidad & Tobago, Turks & Caicos Islands, US Virgin Islands
23rd February:-
Albania, American Samoa, Andorra, Armenia, Azerbaijan, Bosnia & Herzegovina, Cyprus, Estonia, Faroe Islands, Fiji, FYR Macedonia, Georgia, Kazakhstan, Kiribati, Latvia, Lithuania, Luxembourg, Malta, Moldova, Montenegro, New Zealand, Papua New Guinea, Samoa, San Marino, Solomon Islands, Tonga, Vanuatu
8th March:-
Croatia, France, Germany, Greece, Hungary, Israel, N. Ireland, Russia, Scotland, Switzerland, Turkey, Ukraine, Wales
13th March:-
Austria, England
17th March:-
Slovakia
18th March:-
Belgium, Czech Republic, Demark, Italy, Poland, Portugal,
19th March:-
Bulgaria, Romania, Slovenia, Uruguay
23th March:-
Mexico
25th March:-
Holland
28th March:-
Serbia, Spain
2nd April:-
Argentina, India, South Africa
17th April:-
Indonesia
19th April:-
Hong Kong
27th April:-
Australia
7th June:-
Bolivia, Ecuador, Paraguay, Venezuela
13th August:-
Iceland
27th August:-
Afghanistan, Bahrain, Bangladesh, Bhutan, Brunei, Cambodia, Chinese Taipei, East Timor, Guam, Iran, Iraq, Japan, Jordan, Kuwait, Kyrgyzstan, Laos, Lebanon, Macau, Maldives, Mongolia, Myanmar, Nepal, North Korea, Northern Mariana, Oman, Pakistan, Palestine, Philippines, Qatar, Saudi Arabia, Sri Lanka, Syria, Tajikistan, Thailand, Turkmenistan, U.A.E., Uzbekistan, Vietnam, Yemen
2nd September:-
Malaysia, Sweden
14th September:-
South Korea
19th September:-
Chile
20th September:-
Brazil, Ireland
25th September:-
Finland
28th September:-
Singapore, Colombia
1st October:-
Norway
3rd October:-
Algeria, Angola, Benin, Botswana, Burkina Faso, Burundi, Cameroon, Cape Verde, Central African Rep., Chad, Congo, Djibouti, DR Congo, Egypt, Equatorial Guinea, Eritrea, Ethiopia, Gabon, Gambia, Ghana, Guinea, Guinea-Bissau, Ivory Coast, Kenya, Lesotho, Liberia, Libya, Madagascar, Malawi, Mali, Mauritania, Mauritius, Morocco, Mozambique, Namibia, Niger, Nigeria, Rwanda, São Tomé &Príncipe, Senegal, Seychelles, Sierra Leone, Somalia, South Sudan, Sudan, Swaziland, Tanzania, Togo, Tunisia, Uganda, Zambia, Zanzibar, Zimbabwe
8th October:-
China
16th October:-
Peru
2nd November:-
Belarus
26th December:-
USA